rtc: sun6i: Add R40 compatible

The R40 has a quite different RTC, with only a single interrupt line, but
two clock outputs. Let's add a compatible.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>